In a standard scanning acoustic microscope, a target is scanned with a high energy multi-megahertz acoustic beam pulsed at kilohertz rates. The beam as it passes through or is reflected from the target is modified in amplitude and/or phase.
The target may be inspected at various internal interfaces for defects by collecting, amplifying and appropriately time-gating a reflected fraction of the input signal. A greater gate delay represents a deeper reflected level in the target.
The most typical displays produced using this gated return signal will show greater amplitude signals where the acoustic probe at the gated depth is more strongly reflected than at other levels. By way of example, a strong reflection will occur if a disbond between two layers of an IC package has created an air gap, air being highly reflective of acoustic waves traveling through a semiconductor medium.
Scanning acoustic microscopes utilizing such information displays have proven to be of great benefit in nondestructive inspection and testing of semiconductor packages and many other commercial articles and laboratory targets. To generate as much information as possible from the sensed acoustic beam, many image enhancements techniques have been developed—colorization of differentiated information, edge enhancement, and so forth. Yet the desire for more and different information about internal details in inspected targets continues to be intense and unabated.